Coordination of steering in a free-trotting quadruped.

Typically, locomotion has been studied by restricting the animal's path and/or speed, focusing on stride and step kinematics. Here we incorporate measurements of the legs and trunk in the support and swing phases, during trotting with various speeds and curvatures. This paradigm releases the animal from the confines of the treadmill and runway into the open space. The diagonal step, a new unit of locomotion, is defined by regarding the line between the two supporting diagonal legs as a frame of reference for the description of the dynamics of the virtual line between the two swinging diagonal legs. This analysis reveals that during free trotting the mouse uses three types of steps: fixating, opening, and closing steps. During progression along a straight path, the mouse uses fixating steps, in which the swinging diagonal maintains a fixed direction, landing on the supporting foreleg; during progression along a curved path the mouse uses opening and closing steps alternately. If two steps of the same type are performed sequentially, they engender an abrupt change of direction. Our results reveal how steering with the swinging diagonal, while using a virtually bipedal gait, engenders the whole repertoire of free-trotting behavior.